[show abstract][hide abstract] ABSTRACT: The control of the magnetism of ultra-thin ferromagnetic layers using an electric field, rather than a current, has many potential technologically important applications. It is usually insisted that such control occurs via an electric field induced surface charge doping that modifies the magnetic anisotropy. However, it remains the case that a number of key experiments cannot be understood within such a scenario. Much studied is the spin-splitting of the conduction electrons of non-magnetic metals or semi-conductors due to the Rashba spin-orbit coupling. This reflects a large surface electric field. For a magnet, this same splitting is modified by the exchange field resulting in a large magnetic anisotropy energy via the Dzyaloshinskii-Moriya mechanism. This different, yet traditional, path to an electrically induced anisotropy energy can explain the electric field, thickness, and material dependence reported in many experiments.
[show abstract][hide abstract] ABSTRACT: A new spinmotive force is predicted in ferromagnets with spin-orbit coupling.
By extending the theory of spinmotive force, we show that a time-varying
electric field can induce a spinmotive force with static and uniform
magnetization. This spinmotive has two advantages; it can be detected free from
the inductive voltage owing to the absence of dynamical magnetization and it
can be tuned by electric fields. To observe the effect, we propose two
experimental setups: electric voltage measurement in a single ferromagnet and
spin injection from a ferromagnet into an attached nonmagnetic conductor.
[show abstract][hide abstract] ABSTRACT: Spin-rotation coupling, which is responsible for angular momentum conversion
between the electron spin and rotational deformations of elastic media, is
exploited for generating spin current. This method requires neither magnetic
moments nor spin-orbit interaction. The spin current generated in nonmagnets is
calculated in presence of surface acoustic waves. We solve the spin diffusion
equation, extended to include spin-rotation coupling, and find that larger spin
currents can be obtained in materials with longer spin lifetimes. Spin
accumulation induced on the surface is predicted to be detectable by
time-resolved Kerr spectroscopy.
[show abstract][hide abstract] ABSTRACT: We propose a method that allows power conversion from DC magnetic fields to
AC electric voltages using domain wall (DW) motion in ferromagnetic nanowires.
The device concept relies on spinmotive force, voltage generation due to
magnetization dynamics. Sinusoidal modulation of the nanowire width introduces
a periodic potential for a DW the gradient of which exerts variable pressure on
the traveling DW. This results in time variation of the DW precession frequency
and the associated voltage. Using a one-dimensional model we show that the
frequency and amplitude of the AC outputs can be tuned by the DC magnetic
fields and wire-design.
[show abstract][hide abstract] ABSTRACT: We predict the enhancement of the spin-rotation coupling due to the interband
mixing. The Bloch wavefunctions in the presence of mechanical rotation are
constructed with the generalized crystal momentum which includes a gauge
potential arising from the rotation. Using the eight- band Kane model, the
renormalized spin-rotation coupling is explicitly obtained. As a result of the
renormalization, the rotational Doppler shift in electron spin resonance and
the mechanical torque on an electron spin will be strongly modulated.
[show abstract][hide abstract] ABSTRACT: The spinmotive force associated with a moving domain wall is observed directly in Permalloy nanowires using real time voltage measurements with proper subtraction of the electromotive force. Whereas the wall velocity exhibits nonlinear dependence on magnetic field, the generated voltage increases linearly with the field. We show that the sign of the voltage reverses when the wall propagation direction is altered. Numerical simulations explain quantitatively these features of spinmotive force and indicate that it scales with the field even in a field range where the wall motion is no longer associated with periodic angular rotation of the wall magnetization.
[show abstract][hide abstract] ABSTRACT: Spinmotive force induced by domain wall motion in perpendicularly magnetized
nanowires is numerically demonstrated. We show that using nanowires with large
magnetic anisotropy can lead to a high stability of spinmotive force under
strong magnetic fields. We observe spinmotive force in the order of tens of
microvolt in a multilayered Co/Ni nanowire and in the order of several hundred
microvolt in a FePt nanowire; the latter is two orders of magnitude greater
than that in permalloy nanowires reported previously. The narrow structure and
low mobility of a domain wall under magnetic fields in perpendicularly
magnetized nanowires permits downsizing of spinmotive force devices.
[show abstract][hide abstract] ABSTRACT: Spinmotive force associated with a moving vortex domain wall is
investigated numerically. Dynamics of magnetization textures such as a
domain wall exerts a non-conservative spin-force on conduction electrons
, offering a new concept of magnetic devices . This spinmotive
force in permalloy nanowires has been detected by voltage measurement
 where magnitude of the signal is limited less than 500 nV.
Theoretically it is suggested that the spinmotive force signal increases
as a function of external magnetic fields. At higher magnetic fields,
however, the wall propagation mode becomes rather chaotic involving
transformations of the wall structure and it remains to be seen how the
spinmotive force appears. Numerical simulations show that the spinmotive
force scales with the field even in a field range where the wall motion
is no longer associated coherent precession. This feature has been
tested in a recent experiment . Further enhancement of the spinmotive
force is explored by designing ferromagnetic nanostructures  and
materials.  S. Barnes and S. Maekawa, PRL (2007).  S. Barnes, J.
Ieda, and S. Maekawa, APL (2006).  S. A. Yang et al., PRL (2009). 
M. Hayashi, J. Ieda et al., submitted.  Y. Yamane, J. Ieda et al.,
[show abstract][hide abstract] ABSTRACT: We study, both experimentally and theoretically, the generation of a dc spinmotive force. By exciting a ferromagnetic resonance of a comb-shaped ferromagnetic thin film, a continuous spinmotive force is generated. Experimental results are well reproduced by theoretical calculations, offering a quantitative and microscopic understanding of this spinmotive force.
[show abstract][hide abstract] ABSTRACT: We study, both analytically and numerically, a spinmotive force arising from
inherent magnetic energy of a domain wall in a wedged ferromagnetic nanowire.
In a spatially-nonuniform nanowire, domain walls are subjected to an effective
magnetic field, resulting in spontaneous motion of the walls. The spinmotive
force mechanism converts the ferromagnetic exchange and demagnetizing energy of
the nanowire into the electrical energy of the conduction electrons through the
domain wall motion. The calculations show that this spinmotive force can be
several microvolts, which is easily detectable by experiments.
[show abstract][hide abstract] ABSTRACT: We formulate a quantitative theory of an electromotive force of spin origin,
i.e., spin-motive force, by the equation-of-motion approach. In a ferromagnetic
metal, electrons couple to the local magnetization via the exchange
interaction. Electrons feel spin dependent forces due to this interaction, and
then the spin-motive force and the anomalous Hall effect appears. We have
revealed that the origin of these phenomena is a misalignment between the
conduction electron spin and the local magnetization.
Journal of Applied Physics 07/2011; 109(7). · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: Injection of spin currents into solids is crucial for exploring spin physics and spintronics. There has been significant progress in recent years in spin injection into high-resistivity materials, for example, semiconductors and organic materials, which uses tunnel barriers to circumvent the impedance mismatch problem; the impedance mismatch between ferromagnetic metals and high-resistivity materials drastically limits the spin-injection efficiency. However, because of this problem, there is no route for spin injection into these materials through low-resistivity interfaces, that is, Ohmic contacts, even though this promises an easy and versatile pathway for spin injection without the need for growing high-quality tunnel barriers. Here we show experimental evidence that spin pumping enables spin injection free from this condition; room-temperature spin injection into GaAs from Ni(81)Fe(19) through an Ohmic contact is demonstrated through dynamical spin exchange. Furthermore, we demonstrate that this exchange can be controlled electrically by applying a bias voltage across a Ni(81)Fe(19)/GaAs interface, enabling electric tuning of the spin-pumping efficiency.
Nature Material 06/2011; 10(9):655-9. · 35.75 Impact Factor
[show abstract][hide abstract] ABSTRACT: The spin-dependent inertial force in an accelerating system under the
presence of electromagnetic fields is derived from the generally covariant
Dirac equation. Spin currents are evaluated by the force up to the lowest order
of the spin-orbit coupling in both ballistic and diffusive regimes. We give an
interpretation of the inertial effect of linear acceleration on an electron as
an effective electric field and show that mechanical vibration in a high
frequency resonator can create a spin current via the spin-orbit interaction
augmented by the linear acceleration.
[show abstract][hide abstract] ABSTRACT: The inverse spin-Hall effect (ISHE) induced by the spin pumping has been investigated systematically in simple ferromagnetic/paramagnetic bilayer systems. The spin pumping driven by ferromagnetic resonance injects a spin current into the paramagnetic layer, which gives rise to an electromotive force transverse to the spin current using the ISHE in the paramagnetic layer. In a Ni81Fe19/Pt film, we found an electromotive force perpendicular to the applied magnetic field at the ferromagnetic resonance condition. The spectral shape of the electromotive force is well reproduced using a simple Lorentz function, indicating that the electromotive force is due to the ISHE induced by the spin pumping; extrinsic magnetogalvanic effects are eliminated in this measurement. The electromotive force varies systematically by changing the microwave power, magnetic-field angle, and film size, being consistent with the prediction based on the Landau–Lifshitz–Gilbert equation combined with the models of the ISHE and spin pumping. The electromotive force was observed also in a Pt/Y3Fe4GaO12 film, in which the metallic Ni81Fe19 layer is replaced by an insulating Y3Fe4GaO12 layer, supporting that the spin-pumping-induced ISHE is responsible for the observed electromotive force.
Journal of Applied Physics 05/2011; 109(10):103913-103913-11. · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: In the frontier of spintronics, much attention is paid on the control and generation of spin currents. Due to the exciting progress of nanomechatrononics, the importance of mechanical manipulation of electron spin will increase. We discuss theoretically effects of mechanical rotation on spin currents using generally covariant Dirac equation with gauge fields in the non-relativistic limit. We derive semi-classical equations of motion for a wavepacket of electrons in two dimentional planes subject to the spin-orbit interaction argumented by a mechanical rotation. We show that a circular spin current is created by the mechanical rotation with a magnetic field. The magnitude of the spin current becomes 10^8 A/m^2 in Pt with the magnetic field 1T and the rotational velocity 1kHz.
[show abstract][hide abstract] ABSTRACT: We study the Pauli-Schrödinger equation in a uniformly rotating frame of reference to describe a coupling of spins and mechanical rotations. The explicit form of the spin-orbit interaction (SOI) with the inertial effects due to the mechanical rotation is presented. We derive equations of motion for a wave packet of electrons in two-dimensional planes subject to the SOI. The solution is a superposition of two cyclotron motions with different frequencies and a circular spin current is created by the mechanical rotation. The magnitude of the spin current is linearly proportional to the lower cyclotron frequency.
[show abstract][hide abstract] ABSTRACT: Thermoelectric generation is an essential function in future energy-saving technologies. However, it has so far been an exclusive feature of electric conductors, a situation which limits its application; conduction electrons are often problematic in the thermal design of devices. Here we report electric voltage generation from heat flowing in an insulator. We reveal that, despite the absence of conduction electrons, the magnetic insulator LaY(2)Fe(5)O(12) can convert a heat flow into a spin voltage. Attached Pt films can then transform this spin voltage into an electric voltage as a result of the inverse spin Hall effect. The experimental results require us to introduce a thermally activated interface spin exchange between LaY(2)Fe(5)O(12) and Pt. Our findings extend the range of potential materials for thermoelectric applications and provide a crucial piece of information for understanding the physics of the spin Seebeck effect.
Nature Material 11/2010; 9(11):894-7. · 35.75 Impact Factor
[show abstract][hide abstract] ABSTRACT: In the frontier of spintronics, much attention is paid on the control of spin current. Due to the exciting progress of nanomechatrononics, the importance of mechanical manipulation of electron spin will increase. We discuss theoretically effects of mechanical rotation on spin current using generally covariant Dirac equation in the non-relativistic limit. Coupling between rotation and spin is represented by Levi-Civita connections and spinor connections. By taking the non-relativistic limit, we show these connections introduce SU(2) gauge potentials. We derive spin current and forces acting on electron spins in terms of the SU(2)xU(1) theory. The non-relativistic correspondences of the inertial effects, such as energy- momentum redshifts effects, Sagnac-type effect and spin-rotation coupling are discussed in the context of spintronics.
[show abstract][hide abstract] ABSTRACT: Spin wave resonance in Ni <sub>81</sub> Fe <sub>19</sub>/ Pt thin wire arrays has been investigated using the inverse spin-Hall effect (ISHE). The spin wave in the Ni <sub>81</sub> Fe <sub>19</sub> layer drives spin pumping, generation of spin currents from magnetization precession, and the pumped spin current is converted into a charge current by ISHE in the Pt layer. We found an electromotive force transverse to the spatial and the spin-polarization directions of the spin current. The experimental results indicate that the amplitude of the electromotive force is proportional to the spin wave resonance absorption intensity, enabling the electric measurement of spin wave resonance in nanostructured magnetic systems.
[show abstract][hide abstract] ABSTRACT: Magnetic domain wall motion induced by magnetic fields and spin-polarized electrical currents is experimentally well established. A full understanding of the underlying mechanisms, however, remains elusive. We reported a detail comparison of such motions in the thermally activated subthreshold, or "creep," regime, where the wall velocity obeys an Arrhenius law. Experiment shows the velocity scales in an evidently different way with a driving current and field, proving the nonequivalence of two drives. Scaling theory explains the important features of experiment.